Dry etching of semiconducting and insulating materials by reactive ions is of increasing importance in semiconductor technology as an improvement on wet chemical processes. Three aspects of the dry etching process are identified as follows:
(1) physical sputtering by ions impacting the surface; PA0 (2) chemical etching by reactive neutrals and ions; and PA0 (3) "ion-assisted" etching which occurs in the presence of ion bombardment but at energies where sputtering is negligible.
Three basic types of dry etching processes that involve the above aspects are ion milling, plasma etching, and reactive ion etching.
In ion milling, the main etching process used is almost entirely due to physical sputtering, usually by ions accelerated from a dual-grid ion source.
In plasma etching, the wafers to be etched are placed in a chamber of reactive gas (e.g. CF.sub.4) at a pressure of about 1 torr. A glow discharge (plasma) is ignited by an r.f. induction coil outside the chamber. Reactive species are formed by ionization and dissociation in the plasma and diffuse to the wafer surfaces, which are allowed to float close to the plasma potential. Bombardment of the wafer is from all angles, giving isotropic etching of the wafer by mostly energetic neutral gas atoms, for which chemical etching dominates.
Reactive ion etching (RIE) introduces the capability of anisotropic etching, of extreme importance in forming closely spaced surface features. In this process, both chemical etching and ion-assisted etching dominate, and some physical sputtering may occur. The wafers are placed on the cathode of an rf discharge maintained at a pressure of typically 30 millitorr, with cathode voltage of 600 to 900 V. peak-to-peak. The ion sheath which forms above the cathode provides a region of acceleration of positive ions (e.g. CF.sub.3 +) toward the cathode surface. The ions hit the surface near normal incidence, providing a directionality (anisotropy) of etch rate. The lower pressure assists in maintaining the directionality of the reactive ions.
J. W. Coburn, H. F. Winters and T. J. Chuang in their article entitled "Ion-Surface Interaction in Plasma Etching", Journal of Applied Physics, Vol. 48, No. 8, August, 1977, pp. 3532-3540, describe the use of ion beams for reactive ion etching. Here, a single aperture ion source is used to bombard various substrates of different materials with ion beams of materials such as CF.sub.3 +, and Ar+. The single aperture configuration described is limited to currents in the low microamperes range. Also, the configuration cannot be used to etch large areas uniformly.
The technology of ion sources is of interest in reactive ion etching processes. Ion sources have been employed for many years using multi-aperture, multi-grid accelerator systems. In U.S. Pat. No. 3,156,090 to H. R. Kaufman, issued on Nov. 10, 1964, such a system is shown, which is similar to many sources since used for physical sputtering (ion milling). More recently, D. Pigache describes in an article entitled "A Laboratory Simulation of the Ionospheric Plasma", AIAA Journal, Vol. 11, pp. 129-130, ; February, 1973, a high current density ion source that operates at low ion energy due to a single grid accelerator system. This ion source has been used in ionosphere simulation since it can produce a very dense, low energy plasma. A similar ion extraction means is disclosed in U.S. Pat. No. 3,660,715 to Post where a fine mesh screen or mosaic element 46 is employed.
In U.S. Pat. No. 3,355,615 to Bihan et al, a dual grid accelerator system is shown having a pair of grids with a fine mesh grid added for defining the plasma sheath location.
Although Bihan et al employs a fine mesh screen, it is essentially a dual grid system with a fine mesh screen added to one of the grids. As such, such system does not provide the high current density at low ion energy that is obtainable with the single grid design of Pigache described above.
Another type of dual grid accelerator sysem is disclosed by Margosian et al in U.S. Pat. No. 3,744,247 where a layer of dielectric material is interposed between a metal grid and the chamber in an ion thruster. The effect of the insulator is to produce an accelerating voltage difference similar to that developed in a dual grid system.
Accordingly, it is an object of the present invention to provide a controlled reactive ion etching process, with a minimum of etching due to physical sputtering.
It is another object to provide a reactive ion etching process with an ion beam of high current density and low ion energy.